1. Field of the Invention
The present invention relates to a display apparatus having the function of detecting a position and/or the like of an object (object to be detected) which comes into contact with or comes close to a display surface, and a position detecting method for detecting the position and/or the like of the object to be detected.
2. Description of the Related Art
Techniques for detecting a position and/or the like of an object which comes into contact with or comes close to a display surface of a display apparatus are known. Among them, a typified and popularized technique is a display apparatus having a touch panel. There are touch panels of various types, and a popularized one is a touch panel of a type of detecting capacitance. In this type, when a touch panel is touched with a finger, a change in surface charge of the panel is captured to detect the position and/or the like of an object. By using such a touch panel, the user is possible to perform operation intuitively.
Recently, various techniques of enabling the position and/or the like of an object to be detected without providing such a touch panel on the display surface are proposed. For example, in an organic EL (Electro-Luminescence) display, a technique is proposed, of intermittently performing display (light emission) in light emitting elements for image display disposed in the display surface, during the period in which the light emission stops, accumulating charges according to light reception in the light emitting elements themselves, and reading the stored charge amount. Similarly, for example, in a liquid crystal display, a technique is proposed in which light receiving elements are disposed adjacent to display pixels and, during the period in which display (light emission) stops, light is received by the light receiving elements. By using such a display apparatus, the position and/or the like of an object is detectable on the basis of a captured video image. Therefore, by using such a display apparatus, the position and/or the like of an object is detectable with a simple configuration without providing a part such as a touch panel on the display surface.
However, at the time of capturing a video image and the like of an object in such a display apparatus, the luminance of light received is influenced by the surrounding environment (brightness). Consequently, in the case of providing a display apparatus whose environment easily changes with the function of detecting the position and/or the like of an object such as a portable electronic device in particular, the light receiving conditions in the dark environment and those in the light environment are largely different from each other. It is therefore difficult to detect the position and/or the like of an object under uniform light receiving conditions.
For example, Japanese Unexamined Patent Application Publication No. 2007-25796 proposes the following measure for detecting the position and/or the like of an object. That is, in a liquid crystal display, at the time of displaying an image (moving image or still image) frame by frame, each of frame periods is halved. Then, a backlight is turned off in the first-half period to display no image, and the backlight is turned on and a display signal is simultaneously written to display pixels in the latter-half period of each of the frame periods to display an image. Further, a light reception signal is read from a light receiving element in the first-half period in which the backlight is off and the latter-half period in which the backlight is on. By using an image (first image) obtained by calculating the difference of the light reception signals, the position and/or the like of an object is detected. By the measure, the position and/or the like of an object is detectable regardless of the environment (brightness).
However, in the first image, noise which occurs in the display apparatus (for example, noise caused by light from the backlight and variations in sensitivity of the light receiving elements) is included. There is a case that it is difficult to detect the position and/or the like of an object due to the influence of noise. To address the problem, the Japanese Patent Unexamined Patent Application Publication No. 2007-25796 proposes another measure. That is, in a state where there is no external light and there is no surface reflecting object (that is, in a state where there is no object which is in contact with or is close to the surface), light reception signals are read from light receiving elements in both of the case where a backlight is off and the case where the backlight is on. An image (second image) obtained by calculating the difference of the light reception signals is stored in advance in a memory. At the time of detecting the position and/or the like of an object, each pixel data of the second image is subtracted from each pixel data of the first image, thereby eliminating the noise component from the first image.